Short-arc mercury-vapor high-pressure discharge lamps have been known since about the middle of the 1930's as light sources for projection use. They are characterized by high brightness. They also have high ultraviolet radiation intensity, see the referenced "Zeitschr.f.techn.Physik" ("Journal of Technological Physics"), vol. 11, pages 377-379, article by Rompe and Thouret, "Brightness of Mercury Discharge etc.", (1936). Today, such lamps have a discharge vessel made of quartz glass in which two electrodes, spaced from each other, are sealed. The discharge vessel includes a fill including xenon and mercury. The electrodes and fill are so arranged that, in operation of the lamp, the mercury of the fill provides spectral lines of radiation, including an intense line having a wavelength of about 365 nm. The radiation output, also, includes further radiation from lines below 365 nm wavelength. Such lamps have been used for many years in micro-lithography for the exposure of photo lacquers and photo resists used, for example, in the manufacture of integrated circuits, for irradiating wafers or chips. Further development of optical micro-lithography has recently accelerated and substantial interest resides in increasing the storage density or circuit density of the integrated circuits on a wafer. Additionally, the economics of the manufacturing processes are being improved.
The requirement of increasingly higher circuit and storage densities in modern integrated circuits requires manufacture of smaller and smaller structures. Consequently, as the resolution of the structure depends largely on the wavelength of the radiation used to expose semiconductor wafers, and coatings, such as photo resists, thereon the radiation of the intense mercury line of about 365 nm is being used in modern exposure equipment as suitable imaging apparatus, especially adapted to this wavelength became commercially available recently.
Details of an exposure system, when using radiation having a wavelength of 365 nm from a mercury-vapor lamp, are described in the referenced U.S. Pat. No. 4,732,842, Kira, the disclosure of which is hereby incorporated by reference. The economics of exposure to make integrated circuits is improved by decreasing the exposure time and increasing the power of the lamps used for exposure. Nominal powers in the kilowatt range have become feasible.
Powerful short-arc mercury-vapor high-pressure discharge lamps radiate not only at the desired radiation wavelength of 365 nm but, additionally, emit substantial radiation in shorter ultraviolet (UV) wavelength ranges. Expensive filtering in the radiation path, for example introduced into the illuminating systems, leads to problems. It is not possible to completely filter undesired radiation and, further, filtering also attenuates the radiation of the desired wavelength of 365 nm. Filtering was done for example by UV blocking filters, such as interference filters and color filters.
Radiation below about 330 nm can be absorbed in glass of the optical system, but, if so, causes heating of the optical system. This heat must be removed by suitable cooling systems in order to avoid unacceptable heating of the overall system. Radiation below about 330 nm, further, leads to radiation damage of the optical cements used in the imaging optics, even after only comparatively short time of use. Radiation below about 300 nm leads to radiation damage in the glasses themselves, by solarization.
Radiation below about 280 nm wavelength leads to mutual interaction with the surrounding atmosphere and vapor-like contaminants, for example derived from chemicals and organic solvents used in the manufacture of integrated circuits. This interaction leads to formation of aggressive and reactive ozone and condensable compounds and cracked products, all of which can deposit or precipitate on the surfaces of the optical components and form undesirable coatings.
It is desirable to reduce the technical requirements for UV blocking filters, cooling, and air-conditioning and air-cleaning systems. Furthermore, the expensive downtime required by service and repair on the extremely expensive exposure apparatus should be reduced. Typically, such exposure apparatus is used 24 hours per day. Consequently, the light sources in exposure systems should provide a high output in the desired wavelength of 365 nm, but preferably none, or if so, highly attenuated, interfering shorter wavelength UV radiation.